1. Field of the Invention
The present invention relates generally to wireless communications systems and more particularly, to calibration methods and structures in such systems.
2. Description of the Related Art
Performance of modern wireless communications equipment is typically limited by three factors: power & bandwidth, interference, and the wireless channel itself.
For example, the Federal Communications Commission has imposed power and bandwidth limitations on most communications bands (e.g., the 2.4 GHz ISM and the 5 GHz UNII bands) and these limitations limit the range and data rate of associated devices. Accordingly, systems that employ conventional modem and signal processing technologies may not be able to support future multimedia and other high data rate applications (e.g., streaming video).
In unlicensed communications bands, interference is a serious problem since multiple emitters may share the same frequency in an uncontrolled manner. Interference from other emitters significantly reduces system throughput.
Finally, the wireless channel limits overall performance due to large-scale propagation loss, multipath delay spread and temporal channel variation.
To counter these limitations, spatial processing of multiple antennae has been introduced. Spatial processing improves overall system performance and may be implemented in a variety of wireless devices (e.g., ranging from wireless local area network (WLAN) access points to handheld personal digital assistants (PDA)). To be effective, however, spatial processing requires calibration techniques that compensate for amplitude and phase errors between spatial channels.
A conventional calibration method requires the addition of costly radio frequency hardware to transmit and receive calibration signals. For example, time-division duplex (TDD) systems generally share a plurality of transmitter and receiver hardware but add an additional RF transceiver exclusively for calibration. This added hardware is only used during the calibration process and is powered down during normal operation.
In addition, conventional calibration techniques generally analyze gain and phase imbalances at only one frequency within the signal bandwidth. Because gain and phase errors of broadband wireless devices may vary considerably between spatial channels over the receive or transmit bandwidth, these techniques lead to degraded system performance because of spatial channel mismatch at non-calibrated frequencies.